The placer is a geometry manager from Tk. It provides simple fixed placement of windows, where you specify the exact size and location of one window, called the slave, within another window, called the $master. The placer also provides rubber-sheet placement, where you specify the size and location of the slave in terms of the dimensions of the master, so that the slave changes size and location in response to changes in the size of the master. Lastly, the placer allows you to mix these styles of placement so that, for example, the slave has a fixed width and height but is centered inside the master.

The place method arranges for the placer to manage the geometry of $slave. The remaining arguments consist of one or more option=>value pairs that specify the way in which $slave's geometry is managed. If the placer is already managing $slave, then the option=>value pairs modify the configuration for $slave. The place method returns an empty string as result. The following option=>value pairs are supported:

$master is the reference to the window relative to which $slave is to be placed. $master must neither be $slave's child nor be present in a slaves list that directly or indirectly refers to the $slave.

If this option isn't specified then the master defaults to $slave's owner.

Location specifies the x-coordinate within the master window of the anchor point for $slave widget. In this case the location is specified in a relative fashion as a floating-point number: 0.0 corresponds to the left edge of the master and 1.0 corresponds to the right edge of the master. Location need not be in the range 0.0-1.0. If both x and relx are specified for a slave then their values are summed. For example, "relx=>0.5, x=-2" positions the left edge of the slave 2 pixels to the left of the center of its master.

Location specifies the y-coordinate within the master window of the anchor point for $slave widget. In this case the value is specified in a relative fashion as a floating-point number: 0.0 corresponds to the top edge of the master and 1.0 corresponds to the bottom edge of the master. Location need not be in the range 0.0-1.0. If both y and rely are specified for a slave then their values are summed. For example, rely=>0.5, x=>3 positions the top edge of the slave 3 pixels below the center of its master.

Where specifies which point of $slave is to be positioned at the (x,y) location selected by the x, y, relx, and rely options. Thus if where is se then the lower-right corner of $slave's border will appear at the given (x,y) location in the master. The anchor position defaults to nw.

Size specifies the width for $slave. In this case the width is specified as a floating-point number relative to the width of the master: 0.5 means $slave will be half as wide as the master, 1.0 means $slave will have the same width as the master, and so on. If both width and relwidth are specified for a slave, their values are summed. For example, relwidth=>1.0, width=>5 makes the slave 5 pixels wider than the master.

Size specifies the height for $slave. In this case the height is specified as a floating-point number relative to the height of the master: 0.5 means $slave will be half as high as the master, 1.0 means $slave will have the same height as the master, and so on. If both height and relheight are specified for a slave, their values are summed. For example, relheight=>1.0, height=>-2 makes the slave 2 pixels shorter than the master.

In get-mode the placeInfo method returns a list giving the current configuration of $slave. The list consists of option=>value pairs in exactly the same form as might be specified to the place method. If the configuration of a window has been retrieved with placeInfo, that configuration can be restored later by first using placeInfo in set-mode and setting geometry to gt::Place, which is equivalent to a direct call to place.

It is not necessary for the master window to be the owner of the slave window. This feature is useful in at least two situations. First, for complex window layouts it means you can create a hierarchy of subwindows whose only purpose is to assist in the layout of the owner. The ``real children'' of the owner (i.e. the windows that are significant for the application's user interface) can be children of the owner yet be placed inside the windows of the geometry-management hierarchy. This means that the path names of the ``real children'' don't reflect the geometry-management hierarchy and users can specify options for the real children without being aware of the structure of the geometry-management hierarchy.

A second reason for having a master different than the slave's owner is to tie two siblings together. For example, the placer can be used to force a window always to be positioned centered just below one of its siblings by specifying the configuration

in=>$sibling, relx=>0.5, rely=>1.0, anchor=>'n'

Whenever the $sibling widget is repositioned in the future, the slave will be repositioned as well.

Unlike the other geometry managers (such as the packer) the placer does not make any attempt to manipulate the geometry of the master windows or the owners of slave windows (i.e. it doesn't set their requested sizes).